1. Field of the Invention
The present invention relates to organic thin film transistors, and in particular, to an organic thin film transistor including a gate electrode, a gate insulating film, an organic active layer and a source/drain electrode, or a gate electrode, a gate insulating film, a source/drain electrode and an organic active layer, sequentially formed on a substrate, wherein the gate insulating film is a multi-layered insulator comprising a first layer of a high dielectric constant (k) material and a second layer of an insulating organic polymer compatible with the organic active layer, the second layer being positioned directly under the organic active layer.
2. Description of the Related Art
In recent years, most of thin film transistors (TFT) used for display application consisted of amorphous silicon as the semiconductor, silicon oxide, or silicon nitride as the insulator, and metal electrodes. However, with the recent development of various conductive organic materials, research into developing an organic thin film transistor (OTFT) using an organic material as the semiconductor has been made actively. Since its first development in the 1980s, the OTFT has widened its application into functional electronic devices and optical devices. For example, in the field of liquid crystal displays (LCD), which includes the TFT as switching elements controlling the electric fields, there are many attempts to adopt the OTFT due to its flexibility and easy preparing process. As novel electronic material, the organic semiconductor in the OTFT is superior to its inorganic counterpart (i.e. amorphous silicon) because it has many synthetic routes and can be formed in any shape from fiber to film. Further it shows high flexibility and can be manufactured at a low cost. Therefore, the OTFT using the organic semiconductor such as conducting polymers as an active layer is considered to be advantageous in that the overall manufacture can be achieved by a roll to roll process using a plastic substrate because its active layer can be formed by a printing-process under atmospheric pressure, instead of chemical vapor deposition (CVD) using plasma and requiring high pressure and high temperature, so low-priced TFT could be realized.
But, compared with the amorphous Si TFT, the OTFT exhibits disadvantageously lower charge mobility and higher driving and threshold voltages. In this regard, N. Jackson et al. made an improvement and raised possibility for the OTFT's practical use by achieving a charge mobility of 0.6 cm2·V−1·sec−1 with pentacene active layer (54th Annual device Research Conference Digest 1996). However, the charge mobility achieved by N. Jackson still falls short of the required value, and as well, the OTFT in the prior art requires a driving voltage higher than 100 V and a sub-threshold voltage at least 50 times as high as that of amorphous silicon-TFT. Meanwhile, in U.S. Pat. No. 5,981,970 and Science (Vol. 283, pp822–824), there is disclosed a method of lowering the driving voltage and the threshold voltage in the OTFT by use of a high dielectric constant (i.e. high k) gate insulator, in which the gate insulator is made of an inorganic metal oxide such as BaxSr1-xTiO3 (BST; Barium Strontium Titanate), Ta2O5, Y2O3, and TiO2, or a ferroelectric insulator such as PbZrxTi1-xO3(PZT), Bi4Ti3O12, BaMgF4, SrBi2(Ta1-xNbx)2O9, Ba(Zr1-xTix)O3 (BZT), BaTiO3, SrTiO3, and Bi4Ti3O12. In the OTFT prepared by said method, the gate insulator was prepared by chemical vapor deposition, physical vapor deposition, sputtering, or sol-gel coating techniques and its dielectric constant, k, was 15 or higher. By using this high k insulator, the driving voltage can be decreased to −5V, but the charge mobility still remains unsatisfactory, lower than 0.6 cm2·V−1·sec−1. Further, since the process requires high temperatures of 200–400° C., there is a limit in selecting the type of the substrate and as well, it becomes impossible to adopt a common wet process such as simple coating or printing. U.S. Pat. No. 6,232,157 discloses a method of using polyimide, benzocyclobutene or polyacryl as the organic insulating film, but, the OTFT prepared by the method cannot exhibit device characteristics equal to those of the TFT of inorganic insulator.
In order to improve the performance of thin film electronic devices in the prior art, there were many attempts to adopt a multi-layered gate insulator having two or more layers. For example, U.S. Pat. Nos. 6,563,174 and U.S. Pat. No. 6,558,987 disclose a multi-layered gate insulating film made of amorphous silicon nitride and silicon oxide and a double insulating film made of the same material, respectively, and both of the patents reported that there was a substantial improvement in electrical property of the insulator and crystalline quality of the semiconductor layer. However, these patents are inherently related to the inorganic TFT using the inorganic material, such as amorphous or monocrystalline silicon, and thus cannot be applied in the preparation of the organic semiconductor device.
Recently, many attempts have been made to use the OTFT for various drive devices. However, to realize the practical use of OTFT in LCD or flexible displays using organic EL, not only should a charge mobility increase to the level of 5 cm2·V−1·sec−1 or higher, but also improvement in the driving and threshold voltages of the device should be achieved. In particular, for simplifying the preparation and reducing the cost, it can be desirable for the whole process of preparing the OTFT to be carried out by an all-printing or all-spin method on a plastic substrate. Under the circumstances, there have been many research efforts for developing a method to simplify the preparation of the organic gate insulating film and to increase the charge mobility in the interface between the insulator and the organic active layer. However, satisfactory results have yet to be obtained.
Thus, in this art, it is urgently demanded to develop an organic TFT of a new structure that shows high charge mobility, superior insulating properties, and low driving and threshold voltages, and that can be prepared with ease, for example, by a common wet process.